The present invention relates to a control valve for controlling displacement of a variable displacement compressor used for a vehicular air-conditioner.
A typical variable displacement compressor used in a refrigerant circuit includes a crank chamber; a swash plate, which is tiltably located inside the crank chamber; and pistons, which reciprocate by operation of the swash plate. The inclination angle of the swash plate changes in accordance with the pressure in the crank chamber (crank pressure). Each piston moves by a stroke that corresponds to the inclination angle of the swash plate. The displacement of the compressor varies in accordance with the stroke of the pistons.
A control valve is located in the compressor to adjust the crank pressure. The control valve is, for example, located in a supply passage, which connects a discharge chamber of the compressor to the crank chamber. The control valve adjusts the amount of refrigerant gas supplied to the crank chamber through the supply passage from the discharge chamber in accordance with the pressure (suction pressure) of refrigerant gas drawn into the compressor from an evaporator located in the refrigerant circuit.
The control valve includes a pressure sensing mechanism, a valve mechanism, and a solenoid. The pressure sensing mechanism detects the suction pressure and is displaced in accordance with the pressure. The valve mechanism changes the opening degree of a valve hole by the displacement of the pressure sensing mechanism and adjusts the amount of refrigerant gas that flows through the supply passage. The solenoid controls, by exciting a coil, the opening degree of the valve hole in accordance with the suction pressure. The valve mechanism is generally located at the center of the control valve. The pressure sensing mechanism is located at one end of the valve mechanism and the solenoid is located at the other end of the valve mechanism.
Japanese Laid-Open Patent Publication No. 11-218078 and No. 2000-120912 disclose a control valve in which a solenoid is located at the center of the control valve. In the control valve of the publications, a pressure sensing mechanism is located on one end of the solenoid and a valve mechanism is located on the other end of the solenoid. With this structure, the solenoid can be accommodated in a housing of a compressor. Thus, refrigerant gas that has relatively low temperature and that is drawn into the compressor from the evaporator is introduced in the vicinity of the solenoid. Accordingly, the solenoid, which becomes heated due to excitation of a coil, is cooled. As a result, the electromagnetic force of the coil is prevented from decreasing due to the heat of the coil, and the size of the solenoid is reduced, which reduces the size of the control valve.
In the control valve of the publications, a bellows is used as a member for forming the pressure sensing mechanism. Since the manufacturing cost of the bellows is relatively expensive, the bellows is hindered to decrease the manufacturing cost of the control valve. Therefore, a control valve has been proposed that uses a diaphragm, which is manufactured at a lower cost than the bellows. When the diaphragm is used, the diaphragm is displaced in accordance with the suction pressure and the displacement amount is transmitted to a valve body, which selectively opens and closes a valve hole. Since the relationship between the displacement amount of the diaphragm and the suction pressure is not proportional, the fixing position of the diaphragm with respect to the valve hole needs to be set with high accuracy to obtain desired operating characteristics of the control valve.
However, in the control valve in which the solenoid is located at the center, the distance between the diaphragm and the valve hole is long. Therefore, the number of parts located between the diaphragm and the valve hole is increased. As a result, the fixing position of the diaphragm is displaced from the desired position due to the dimensional error and the assembling error of each part. Accordingly, the control accuracy of the control valve is decreased.